1. Field of the Invention
The invention relates to a micro-electro mechanical device, a micro-electro mechanical device array, a light modulation device, a micro-electro mechanical light modulation device, a micro-electro mechanical light modulation device array, and an image forming apparatus using the same.
2. Description of the Related Art
In recent years, micro-electro mechanical devices which electrically displace and move a micro-mechanical device in the order of μm have been developed briskly because of rapid advancements in MEMS (Micro-Electro Mechanical Systems) technology. Among micro-electro mechanical light modulation devices which have a light modulation function, there is, for example, a DMD (digital micro-mirror device; made by Texas Instruments Inc.) which can tilt a micro mirror to deflection light (see U.S. Pat. No. 6,618,186 and JP 2002-189178 A (which derive from family patent applications). This device exercises an electrostatic force on a movable portion, which is tilted to one side by an electrostatic force, in another direction to thereby rotationally displace the movable portion and modulate light in a mirror part of the movable portion. The DMD is used in wide applications, such as a projection display, a video monitor, a graphic monitor, a television and an electrophotographic printer, in the field of optical information processing. Further, application of optical switches to optical communication, optical interconnection (signal connection technology by means of light, such as an interconnection network in parallel computers), optical information processing (information processing by means of optical operation), etc. has been expected.
A transition time (time from a state where a movable portion is tilted to one side to a state where the movable portion is tilted to another side) or a response speed (speed when a movable portion in a state where the movable portion is tilted to one side is tilted to another side) of a movable portion of a rotational system device of the related art such as the DMD, is determined by the balance among (i) the inertia moment depending on the structure of the movable portion, (ii) the elastic force of a supporting portion, which supports the movable portion so that the movable portion is elastically deformable, and (iii) the magnitude of a voltage to be applied. Appropriate values are adopted at the time of design.
As for the relation between the transition time T and the supporting-portion elastic force K, as shown in FIG. 20, if the supporting-portion elastic force K becomes small, the transition time T of the device tends to increase because the elastic restoring force of the movable portion decreases. It is important to shorten the transition time in order to increase the responsiveness of the device. Therefore, in order to achieve a desired elastic restoring force, a design has generally been made to adopt the supporting-portion elastic force K, which is not excessively small, i.e., to increase the supporting-portion elastic force K.
However, if the supporting-portion elastic force K is increased in order to shorten the transition time T of the device, it is necessary to apply an electrode voltage against the supporting-portion elastic force K in order to drive the device. Thus, a driving voltage increases. Also, if the driving voltage increases, the electric power for the driving of the device will increase. Accordingly, it would be necessary to make a power supply device larger. For this reason, there is a trade-off relation between the followings. That is, in order to control the device in power-saving driving, the transition time becomes long, which leads to sacrifice of high-speed driving, and in order to control the device in high-speed driving, the supporting-portion elastic force K will increase, which leads to sacrifice of the power-saving driving.
Further, even if the supporting-portion elastic force K is increased, the amount of change in the transition time T is slight. Also, even if the supporting-portion elastic force K is increased to shorten the transition time T, a large amount of change in the transition time T cannot be expected in actuality.